Exopolymer production by the green alga penium margaritaceum: Implications for biofilm residency

Penium margaritaceum (Streptophyta) is a desmid that is commonly encountered in biofilms of wetlands of the Adirondack region of New York. This alga was isolated and has become an important model organism for laboratory studies dealing with the production and secretion of the extracellular polymeric substance (EPS) and biofilm behavior of desmids. Penium margaritaceum's EPS macromolecules are used for three general biofilm-based activities: the initial adhesion to a substrate, gliding motility along the substrate, and ensheathment within the biofilm matrix. EPS production is stimulated in experimental cultures by increasing the amounts of sodium nitrate or substituting sodium nitrate with ammonium salts. EPS levels decrease when cells are grown at 30 degrees C or under high calcium levels. Ultrastructural and immunogold-labeling studies show that the EPS is produced in a well-defined and highly ordered endomembrane system. EPS is initially processed in the Golgi bodies and carried in osmiophilic vesicles to the peripheral cytoplasm, the site of active cytoplasmic streaming. The EPS vesicles fuse at specific loci of the plasma membrane and release their constituents to channels within the cell wall. The EPS is ultimately deposited on the cell surface. This secretory system allows the desmid to have a mobile supply of EPS ready for secretion at all times.